ABSTRACT The direct‐feed high‐level waste (DFHLW) vitrification process at the Waste Treatment and Immobilization Plant will process millions of gallons of nuclear waste stored underground at the Hanford site. This study examines DFHLW melter feeds, with glass yields ranging from 222 to 754 g glass L −1 waste, investigating their melting behavior through feed expansion, evolved gas analysis, and thermal characterization. Water content was shown to have the largest effect on total conversion enthalpy and glass production rates. While foam onset temperature, bottom temperature, and total volume varied between the feeds, the impact on the final predicted glass production rate was considerably less than the impact of water content and resultant glass yield. These findings improve understanding melter feed performance based on glass yield and support refinement of models, including melt rate correlation for optimizing melter operation. Future scaled testing is recommended to validate results and optimize vitrification throughput.
We designed, synthesized, and characterized 26 cementitious calcium-magnesium aluminosilicate (CMAS) glasses using the chemical and mineralogical properties of ordinary Portland cement (OPC), ground, granulated blast furnace slag (BFS), and class F fly ash as bounding conditions. The compositions ranged from 31.3-43.8 mol% SiO2, 2.8-14.3 mol% Al2O3, 32.1-52.1 mol% CaO, and 0.0-23.1 mol% MgO. The designed compositional space closely resembled that of reported BFS compositions. Our purpose for this design was to create glasses that would react with aqueous simulated low-level radioactive waste (LLW) solutions upon contact and form solid phases over time. The glasses' structure was determined via Raman spectroscopy and was correlated with exothermic heat release measured by isothermal calorimetry. We observed a moderate to strong correlation between an increasing degree of depolymerization and increasing heat flow (R 2 = 0.66-0.78). After the completion of the isothermal calorimetry experiments, we examined the resulting solidified materials and observed no free-standing liquid-an important property for low-level waste disposition. Powder X-ray diffraction patterns collected on five representative solids, approximately 3 months after completion of the calorimetry experiments, contained reflections consistent with calcium-silicate hydrate and layered double hydroxide phases, supporting-though not conclusively demonstrating-that cementitious reactions occurred between the glasses and the simulated LLW.
The vitrification of high-level waste (HLW) into borosilicate glass is a cornerstone of nuclear waste management, yet one of the key challenges in glass formulation for high-aluminum-containing waste streams is the precipitation of nepheline during the canister cooling stage. Excessive nepheline formation compromises glass durability, threatening the long-term stability of vitrified waste. Thus, predictive models capable of accurately anticipating nepheline formation from glass composition are critical for waste glass formulation and optimizing stability and durability. Various methods such as discriminators and sub-mixture models have been developed to determine nepheline precipitation; however, they tend to be overly conservative and unnecessarily limit achievable glass compositions. Building on previous works, we explore the application of several classical machine learning models-Gaussian Processes, Gradient Boosting, and Random Forest-alongside Bayesian methods, including fully Bayesian Gaussian Processes and Partial Bayesian Neural Networks, to address the challenges posed by limited datasets and prediction uncertainties intrinsic to this problem. Furthermore, we introduce a dual-head partially Bayesian neural network with scaled regression loss, validated through ablation, to directly predict nepheline precipitation. Notably, this regression-based strategy enables the use of practical nepheline tolerance thresholds, substantially reducing false positives compared with existing binary nepheline discriminator approaches that cannot distinguish between trace and significant nepheline formation-expanding access to novel compositional regions for waste glass design. By balancing explainability, uncertainty quantification, and accuracy, our results lay the foundation for more efficient optimization of HLW vitrification processes and contribute to the broader goal of improving nuclear waste management strategies.
The controlled crystallization of Cs+/Sr2+/Na+-admixed aluminosilicate glasses with a mineral-like composition, based on coal fly ash cenospheres, was studied as a simulation of the immobilization of 137Cs and 90Sr in glass-crystalline matrices. The incorporation of Cs+ and Sr2+ in mineral-like phases using cenospheres with SiO2/Al2O3 ratios of 1.5-3.4, characteristic of Cs+- and Sr2 +-bearing minerals, was demonstrated. Cs+, Sr2+, and Na+ immobilization was achieved by mixing the cenospheres with predetermined ratios of CsNO3/Sr(NO3)2/NaNO3 solutions, followed by drying and calcination at temperatures between 750 degrees C and 1100 degrees C. Cs+ immobilization via this method resulted in glass-ceramics incorporating up to 87 wt% of the Cs+-phases pollucite and kalsilite. Processing the Sr(NO3)2/cenosphere mixtures results in the formation of mineral-like phases of Sr-anorthite and Sr-gehlenite. The Na+-induced transformation of the cenosphere glass was performed to simulate the solidification of 137Cs-bearing sodium-enriched radioactive waste, with nepheline acting as the host phase for 137Cs. Depending on the sodium loading and temperature, the NaNO3/cenosphere blends transform into glass-ceramics with different glass phase/nepheline ratios. The cesium-normalized leaching rates from the glass-ceramics were found to be in the range of 10-7-10-9 g cm-2 day-1 by conducting the chemical durability test. These rates meet the requirements of GOST R 50926-96 and NP-019-2000 (Russia). This demonstrates the high durability of cenosphere-based glass-ceramics, facilitating their safe geological disposal.
High-alumina silicate glasses with varying Gd2O3 content were prepared by a conventional melt-quenching method to investigate the effect of Gd2O3 incorporation on the structure, transmittance, microhardness, and chemical strengthening performance of the SiO2-Al2O3-Na2O-K2O-MgO glass system. The results show that the incorporation of Gd2O3 in the high-alumina silicate glasses enhances structural compactness, T g, and microhardness of the baseline glass, although slight depolymerization of the glass network is induced. The measured visible light transmittance of the glasses with and without Gd2O3 is about 90% at 550 nm. With Gd2O3 doping, the microhardness of the glasses increases up to 7.28 GPa, and the surface compressive stress (CS) reaches 1064 MPa. Moreover, the potential mechanisms of the improved performances are analyzed and discussed. These results provide a theoretical basis for understanding the structural role of Gd2O3 in high-alumina silicate glass and for designing high-performance aluminosilicate glasses.
